NATURAL CAUSES: An Epidemic of Wellness, the Certainty of Dying, and Killing Ourselves to Live Longer, by Barbara Ehrenreich. Twelve, 234 pp., $27.

Barbara Ehrenreich wants you to know that you are going to die. Get used to it, and get beyond it.

That’s the central message of Ehrenreich’s new book, “Natural Causes: An Epidemic of Wellness, the Certainty of Dying, and Killing Ourselves to Live Longer.” Ehrenreich, an investigative journalist, memoirist and author of 23 books (including “Nickel and Dimed”), mounts an intellectual assault on America’s obsession with youth, anti-aging and the denial of death. It’s short (209 pages of text), more a collection of linked essays than a complete work of reportage. It’s a form that creates some problems, though it doesn’t obscure the final and most pertinent message.

Ehrenreich has an unusual combination of tools at her disposal — she’s a polemicist who is also a scholar and scientist (she has a Ph.D. in cellular immunology). The polemicist dominates the early chapters, in which she lacerates the contemporary obsession with aging well and putting off death indefinitely.

Targets include unnecessary medical tests, notably annual physical exams. The fitness craze is an easy mark — though she herself is a gym rat, Ehrenreich looks askance at workouts that suck hours of valuable time out of a person’s day and, longevity wise, have a low cost-benefit ratio. Ever alert to issues of class in America, Ehrenreich writes that “working out is another form of conspicuous consumption: Affluent people do it and, especially if muscular exertion is already part of their job, lower-class people tend to avoid it.” Another longevity-based regime, “wellness” and its subset, “mindfulness,” have little scientific basis and have become profit centers for predatory corporations.

Each of these early chapters could be a book, and in the name, perhaps, of moving things along, sometimes Ehrenreich attempts a rhetorical knockout punch. She writes of the war on smoking: “As more affluent people gave up the habit, the war on smoking — which was always presented as an entirely benevolent effort — began to look like a war on the working class.”

How can eliminating lung cancer be bad for the working class? Ehrenreich targets exorbitant cigarette taxes and the decline of working-class smoking places (break rooms, cigarette-friendly bars) as evidence. Certainly these anti-smoking strategies have outsized impacts on poor people, but lung cancer is lung cancer, no matter how much or little money you’re making. These verbal left hooks distract from more astute observations, such as the likely reasons for working-class smoking: for the economically struggling, surviving in our winner-take-all social system is a prescription for 24-hour stress.

But Ehrenreich is an inspired science writer, and in the latter part of the book she deflates the concept that humans can ever control their biology and their fate.

Her Exhibit A is the macrophage. It’s a cellular entity once thought of as one of the body’s happy warriors, dedicated to cleaning up diseased or broken cells. In recent years macrophages have been exposed as biological double agents for their role in cancer and devastating autoimmune diseases.

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These cellular activists have a disturbing tendency to go their own way. They were once thought to mass at tumor sites for an assault on the tumor’s growth, but scientists have discovered that instead, they encourage the cancer cells “to continue on their reproductive rampage.” Macrophages have been implicated in inflammatory diseases such as atherosclerosis, arthritis, Alzheimer’s disease, diabetes and osteoporosis.

Macrophages upend the premise that the body is a “smooth-running machine in which each part obediently performs its tasks for the common good,” Ehrenreich writes. “It is at best a confederation of parts — cells, tissues, even thought patterns — that may seek to advance their own agendas, whether or not they are destructive of the whole.”

And here is her most salient point. Cells and viruses and subatomic particles don’t have “consciousness, desires or personalities. What they possess is agency, or the ability to initiate an action.” The natural world is not dead, but “swarming with activity, sometimes perhaps even . . . intentionality.”

Ehrenreich’s complex explanation boils down to a simple prescription, though the medicine may be hard to take: “You can think of death bitterly and with resignation . . . and take every possible measure to postpone it,” she writes. “Or, more realistically, you can think of life as an interruption of an eternity of personal nonexistence, and see it as a brief opportunity to observe and interact with the living, ever-surprising world around us.”